Nanostructured bacteria-resistant polymer materials

ABSTRACT

Methods for creating nanostructured surface features on polymers and polymer composites involve application of low pressure during curing of solid polymer material from a solvent solution. The resulting nanoscale surface features significantly decrease bacterial growth on the surface. Polymer materials having the nanoscale structuring can be used in implantable medical devices to inhibit bacterial growth and infection.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority of U.S. Provisional Application No.61/877,063 filed 12 Sep. 2013 and entitled “Nanostructuring of Polymersand Polymer Composites Using Pressure”, the whole of which is herebyincorporated by reference.

BACKGROUND

Several techniques have been used to improve the cytocompatibility ofpolymer surfaces by chemical or physical techniques for tissueengineering applications. For example, polyurethane (PU) and poly-lacticco-glycolic acid (PLGA} surfaces can be nanostructured by etching apolymer surface with solutions of HNO₃ and NaOH, respectively. Thapa, A,et al., Biomaterials (2003) 24:2915-2926; and Thapa, A, et al., J.Biomed. Mater. Res. (2003) 67A:1374-1383. Nanostructuring of polymersurfaces can improve cell adhesion and proliferation. Additional methodsof nanostructuring polymer surfaces include electron beam lithography,mechanical brushing (e.g., U.S. Pat. No. 7,838,074), nanopatterning thesurface of a metal mold (e.g., U.S. Pat. No. 8,168,076), ion beametching, reactive ion etching, plasma etching, and plasma assistedchemical vapor deposition (PACVD) (e.g., U.S. Pat. No. 8,486,280).

Medical devices that are surgically implanted are subject to bacterialgrowth and biofilm formation on their outer surfaces, which oftenresults in serious infections and the need to surgically remove orreplace the device. Previous efforts to inhibit the growth of bacteriaon such surfaces include chemical modification of the polymer surface(e.g., U.S. Pat. No. 4,001,432), and the application of coatingscontaining antimicrobial materials such as silver (e.g., U.S. Pat. No.8,192,765). However, such methods remain subject to variability, poordurability, and potential toxicity.

Thus, there remains a need to develop new methods of nanostructuringpolymer surfaces used in implantable medical devices and developingsurfaces that resist bacterial adhesion and biofilm formation.

SUMMARY OF THE INVENTION

The present invention provides methods for creating nanostructuredsurface features on polymers and polymer composites using low pressure(i.e., vacuum). The methods of the invention create nanostructuredsurface features on polymers and polymer composites using low pressureconditions. Specifically, by allowing polymers to cure (solidify) underlow pressure, distinct polymer nano features can be created whichsignificantly alter cellular functions (such as decreasing bacterialgrowth). This is because nanostructured surface features alter surfaceenergetics to control initial protein adsorption events important foraltering cellular functions. This technology can be used while a polymeris being extruded, cast-mold, or after production in which small amountsof a solvent are added to the polymer as long as pressure is beingapplied.

The invention allows the creation of nanoscale surface features onpolymers using pressure during the solidification process. The resultingnanoscale surface can control cell functions, such as decreasingbacterial responses to an implant bearing the material on its surface,without the use of drugs such as antibiotics or other antimicrobialagents. The methods of the invention involve simple application of lowpressure in a process that can be used during medical device productionor after medical device function.

The invention can be used to decrease bacterial adhesion andcolonization by altering nanoscale surface roughness alone, therebyavoiding pharmaceutical agents or other bioactive agents which may haveside effects and can complicate regulatory approval. Methods of theinvention for introducing bacteriostatic nanostructuring of polymersurfaces are simple and inexpensive, and involve a process that can beused during medical device production or following medical deviceproduction. The technology will significantly enhance the usefullifetime of an implanted medical device. Examples of implantable medicaldevices that can utilize the technology include catheters, endotrachealtubes, orthopedic implants, vascular stents, pacemaker leads, cartilageimplants, and any implants that employ polymers or polymer composites ona surface and are subject to causing infection.

One aspect of the invention is a method of nanostructuring a surface ofa polymer material. The method includes the steps of: (a) providing asolution containing a polymer material dissolved in an organic solvent;and (b) evaporating the solvent at low pressure, whereby the polymermaterial solidifies into a solid form containing the polymer material.The low pressure applied in step (b) is preferably in the range fromabout 0 to about 5 millibars (mbar), i.e., less than about 5 mbar, andmore preferably about 2 mbar or less than about 2 mbar, or even about 1mbar or less than about 1 mbar. As a result of the method, a surface ofthe polymer material becomes nanostructured. The surface roughness is onthe nanoscale (i.e., having features ranging in size from about 1 nm toabout 999 nm or less than 1000 nm), and in preferred embodiments thefeatures have maximum dimensions characterized by a root mean square(RMS) size of about 100 nm or less, or between 0 nm and 100 nm, or about50 nm or less, or between about 0 nm and 50 nm. The nanostructuringprovides the useful function of inhibiting the adhesion of bacteria tothe surface, including pathogenic bacteria that can cause infections asa result of implanting a medical device. Such bacteria includePseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, andStaphylococcus epidermidis.

The method can be performed using any polymer that can be dissolved in asolvent. Preferred polymers include poly-lactic co-glycolic acid (PLGA),polyurethane, polycaprolactone, poly-ether-ether-ketone (PEEK),polyethylene, polypropylene, polypyrrole, and polystyrene, as well asmixtures (i.e., co-polymers) thereof and polymer composites containingthese polymers or mixtures thereof. Any solvent or mixture of solventscan be used that is capable of dissolving the polymer and capable ofremoval by evaporation under low pressure conditions. Preferred solventsinclude chloroform, dimethyl formamide (DMF), acetone, chlorobenzene,cyclohexanone, cyclopentanone, dimethyl sulfoxide (DMSO), nitrobenzene,1, 2-dichlorobenzene, diisopropyl ketone, dioxane, ethylene chloride,isophorone, toluene, and mesityl oxide.

Another aspect of the invention is another method of nanostructuring asurface of a polymer material. The method includes the steps of: (a)providing an article or device containing a polymer material surface andan organic solvent capable of dissolving the polymer material; (b)applying the solvent to the polymer material surface, whereby a portionof the polymer material surface dissolves in the solvent; and (c)evaporating the solvent at low pressure, whereby the dissolved polymermaterial solidifies into a solid form comprising the polymer material.As a result of performing the method, a surface of the polymer materialbecomes nanostructured.

Yet another aspect of the invention is yet another method ofnanostructuring a surface of a polymer material. The method includes thesteps of: (a) providing a solution containing a melt of a polymermaterial, the melt containing a solvent that is volatile at an extrusiontemperature of the melt; (b) extruding the melt from an extrusion deviceto form an extruded article containing the polymer material; and (c)evaporating the solvent from the extruded article at low pressure. As aresult of performing the method, a surface of the polymer materialbecomes nanostructured.

Still another aspect of the invention is a polymer material havingnanostructure on at least a portion of its surface. The nanostructuringis produced by a method including any of the previously describedmethods.

Even another aspect of the invention is a polymer material havingnanostructuring on at least a portion of a surface of the material. Thenanostructured surface contains protruding surface features having anRMS height, width, and/or length of about 100 nm or less.

Another aspect of the invention is an article of manufacture or a devicecontaining any of the previously described nanostructured polymermaterials 34 at a surface of the article or device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows nanoscale roughening of PLGA using low pressure (1 mbar)during solidification over 24 hours. The average feature size was:25 nmdiameter and 10 nm high. FIG. 1B shows lack of such nanoscale surfacestructure when solidification was performed at ambient conditions (1bar). Scale bar=1 μm.

FIG. 2 shows decreased P. aeruginosa colony forming units onnanostructured PLGA. The y-axis is in units of 10⁷ colony forming units.The data shown are mean±SEM, n=3, *p<0.01 compared to plain (non-vacuumtreated and unstructured) PLGA.

FIG. 3A shows nanoscale structuring of PEEK using low pressure (1 mbar)during solidification over 24 hours. The average feature size: was 45 nmdiameter and 30 nm high. FIG. 3B shows surface structure whensolidification was performed at ambient conditions (1 bar). Scalebar=500 nm.

FIG. 4A shows nanoscale structuring of polyurethane using low pressure(0.5 mbar) during solidification over 48 hours. The average featuresize: was 10 nm diameter and 5 nm high. FIG. 4B shows surface structurewhen solidification was performed at ambient conditions (1 bar). Scalebar=1 micron.

FIG. 5A shows nanoscale structuring of polyethylene using low pressure(1.5 mbar) during solidification over 12 hours. The average featuresize: was 10 nm diameter and 5 nm high. FIG. 5B shows surface structurewhen solidification was performed at ambient conditions (1 bar). Scalebar=1 micron.

FIG. 6 shows the decrease in bacterial counts on nanotextured polymersurfaces. Data shown are mean+/−SEM, n=3. All values were significantlysmaller on the nanomodified polymer surfaces compared to unmodifiedpolymer surfaces at p<0.01. Time of culture was 24 hours. The y-axisshows units of 10⁷ colony forming units.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides methods which utilize low pressure conditions tocontrol solvent evaporation during solidification or curing of a polymerpolymer material, which results in nanostructuring of the materialsurface, or at least a portion of the material surface. Whenappropriately controlled, this curing process can create nanoscalesurface features on polymers and polymer composites that are commonlyused in implantable medical devices. During the application of the lowpressure conditions (which can be adjusted to different polymers orpolymer composites), the polymer slowly solidifies and forms surfaceswith nanoscale surface features or roughness. These nanoscale surfacefeatures then in turn alter the surface energy of the material or adevice made using the material. The change in surface energy in turn isbelieved to change the initial protein adsorption events that occur whenthe material is implanted into the body, thereby altering cellularresponses to the implanted material or device. The result is a decreasein bacterial growth on the surface and an inhibition of biofilmformation on an implanted device containing the material. The technologycan be used for any polymer and can be incorporated into procedures usedduring medical device fabrication, such as electro-spinning, extrusion,or molding. The methods of the present invention also can be used as apost-treatment following medical device fabrication.

In order to carry out a method according to the present invention, atleast a portion of the polymer material is dissolved in a solvent. Theconcentration of dissolved polymer in the solvent can be anyconcentration consistent with the method of forming an article or devicefrom the polymer. The polymer can be completely dissolved in thesolvent, or the polymer can be only slightly or partially dissolved inthe solvent, or in the case of a polymer melt, the solvent can bedissolved in the molten polymer. The solution or melt containing thepolymer material and solvent can be placed into a mold or an extrusiondevice. A low-pressure condition is then applied to evaporate thesolvent. As the solvent evaporates the polymer material begins tosolidify into a solid form. During the final stages of solventevaporation, nanostructuring of the solid form surface occurs asresidual solvent leaves the surface of the forming polymer material. Theresult is a nanostructured polymer surface. The method can be carriedout using practically any polymer material or copolymer material or evena composite material containing polymer material. A suitable solventmust be used which allows for the polymer material to be dissolved atleast partially, and also allowing removal of the solvent by evaporationunder suitable conditions of temperature and pressure.

Variations of the method also may be performed, wherein solvent, or asolvent-polymer solution, is applied to an already formed solid polymermaterial such that a portion of the polymer material is dissolved in thesolvent, or such that a new layer of polymer material is laid down onthe surface of the polymer material when the solvent evaporates. Thesolid form coated with solvent-polymer solution is then placed underappropriate vacuum conditions for evaporation of the remaining solvent.Yet another variation of the method can be performed wherein a polymermelt containing a small amount of solvent is placed into an extrusiondevice. For example, the polymer melt can contain 10% or less, 5% orless, 1% or less, or efen 0.1% or less of solvent by weight. Thematerial is extruded from the device and then the extruded object isplaced under low-pressure conditions under which the remaining solventevaporates and creates nanostructure at the material surface.Alternatively, a freshly extruded solid object can be coated withsolvent during its curing process, and placed into a vacuum chamber.

Structures formed at the surface of the polymer material can vary in therange from the nanoscale (from 1 to 999 nm) to the microscale (from 1 to999 μm); however, structures in the nanoscale range are of interest interms of their effects on surface energy and protein and cellularadsorption to the surface. Nanoscale surface structures can have variousgeometries, but generally take the form of approximately spherical orhemispherical structures having a diameter of about 100 nm or less, or50 nm or less, or from 1 to 10 nm, or from 1 to 20 nm. The height ofsuch structures may range, for example, from 1 to 100 nm, from 1 to 20nm, from 1 to 10 nm, or from 1 to 5 nm. Nanoscale surface structures maytake various forms on a given surface and may show a range of size andform distribution. For example, size distribution of nanoscale featuresmay have distribution which can be described as having a root meansquare (RMS) value from 1 to 100 nm, from 1 to 50 nm, from 1 to 20 nm,less than 100 nm, less than 50 nm, or less than 20 nm, for example. Insome embodiments the RMS feature size may be 5 nm or less, or 10 nm orless. In certain embodiments the vertical height above the surface maybe less than the lateral size of surface features. In general, surfacefeatures which have the ability to inhibit binding of bacterial cells tothe surface are preferred.

The invention can be practiced over a range of low absolute pressures,i.e., vacuum pressures well below one atmospheric pressure, such aspressures of 20 mbar or less, 10 mbar or less, 5 mbar or less, 2 mbar orless, about 1 mbar, or less than 1 mbar. Suitable pressures can be foundby testing different solvent temperature and viscosity conditions, andmay need to be adjusted for different polymers. Factors that may beimportant for adjusting the formation of nanostructures while carryingout a method according to the invention include temperature, rate ofchange of temperature, the viscosity of the solvent-polymer solution,concentration of polymer in solution, time of evaporation, pressureduring evaporation, and pressure gradients. For example, a suitablerange of temperature during the evaporation process is from 0 to 70° C.,a suitable rate of change of temperature is from 0 to 20° C. per minute,a suitable range of viscosity is from 10⁻² to 10¹² poise, a suitableconcentration of the polymer in organic solvent solution is from 0 to99% by weight, a suitable time of evaporation is up to one week,suitable pressure gradients include 0-10 mbar, and a suitable rate ofpressure change is from 0 to 5 mbar per minute.

Any type of polymer, copolymer, or polymer composite material can beused with a method of the invention. Preferred polymers include PEEK,PU, PE, and PLGA. Composite materials can include inclusions such ascarbon fiber, nanotubes (e.g., single-walled or multi-walled carbonnanotubes), nanoparticles, crystals, or other filler or reinforcingmaterials. Such materials can be added, for example, to thesolvent-polymer solution.

Without intending to limit the invention to any particular mechanism, itis believed that the effect of nanoscale surface features on bacteria ismediated through alteration of the surface energy of the polymermaterial, which affects the adsorption of protein and bacterial cells tothe surface. The size of the nanoscale features may be relevant indifferentiating between bacterial cells, which should be excluded, andmammalian cells which need to adhere to the device for its effectiveuse. Methods of the invention can be adjusted to provide desirednanoscale features on a polymer surface, and the effect of such featureson bacterial adsorption and colonization of the material can be testedusing bacterial growth on the material in culture.

A variety of medical devices utilizing the nanostructured polymermaterials can be implanted into the living body of a subject. Thesubject can be, for example, a human patient, a mammal, or anotheranimal. The device can either be entirely fabricated out of polymermaterial, or the polymer material can be used to form a portion of thedevice such as a coating on the surface of the device, or to form anexposed substructure of the device. While the nanostructuring of theinvention can be employed on any medical device, particularly medicaldevices that are implanted, the nanostructuring can be especially usefulon devices that are subject to infection when implanted. Preferreddevices for use with the nanostructuring of the invention includecatheters, endotracheal tubes, orthopedic implants, vascular stents,heart valves, pacemaker leads, cartilage implants, and bone implants.

The invention also can be used in the production of other articles ofmanufacture, including hand-held consumer articles which are notintended for medical use, such as switches, knobs, buttons, writingimplements, or touch-screens, particularly where there is a desire toprevent bacteria and/or protein from adsorbing to the surface of thearticle.

EXAMPLES Example 1: Nanostructuring of a PLGA Polymer Surface

Poly-lactic-co-glycolic acid (50:50% PLA:PGA; Polysciences) was pouredinto a Petri dish after being dissolved in chloroform. The soluble PLGAwas then placed in a pressure chamber to allow the polymer to solidifyat two mbar absolute pressure and 26° C. for 48 hours. The low pressurecondition was established and maintained by continuous operation of avacuum pump. SEM images demonstrated a nanostructured surface for thePLGA allowed to solidify at low pressure (FIG. 1A), while images of thesame PLGA-chloroform solution allowed to solidify under ambientconditions (1 bar absolute pressure) showed a smooth surface (FIG. 1B).

Example 2: Inhibition of Bacterial Growth on a Nanostructured PLGASurface

A bacterial cell line of Pseudomonas aeruginosa (P. aeruginosa) wasobtained in freeze-dried form from the American type culture collection(catalog number 27853; Manassas, Va., USA). The bacterial cells werepropagated in 30 mg/mL tryptic soy broth (TSB) (catalog number 22092,Sigma-Aldrich, St. Louis, Mo., USA) for 24 hours in an incubator (37°C., humidified 5% carbon dioxide). When the second passage of bacteriareached its stationary phase, the second passage was frozen in one partTSB and one part 40% sterile glycerol. Before bacterial seating, asterile 10 μL loop was used to withdraw the bacteria from the preparedfrozen stock and streaked onto a TSB agar plate, and the TSB agar platewas incubated for 20 hours to form single bacterial colonies. Thebacterial solution was diluted to a concentration of 10⁷ bacteria/mL,which was assessed by measuring the optical density of the bacterialsolution using a standard curve correlating optical densities andbacterial concentrations. The optical densities were measured at 562 nmusing a SpectraMax M5 plate reader (Molecular Devices, Sunnyvale,Calif.).

Bacteria (10⁵ bacteria) were then seeded onto the samples of interestfor 24 hours. At that time, bacteria were removed, diluted with a PBSsolution, and were spread on agar plates where bacterial colonies werecounted. Bacterial growth tests were conducted in triplicate andrepeated three times. Data were collected and differences were assessedwith the probability associated with a one-tailed Student's t-test.

The results showed significantly fewer bacteria on the nanostructuredPLGA compared to plain (untreated) PLGA (FIG. 2).

Example 3: Comparison of Nanostructuring of Different Polymer Materials

Nanostructuring was performed on several different polymer materialsusing a procedure similar to that described in Example 1. Each materialshowed nanostructuring after the solvent evaporation process was carriedout at low pressure.

Polyether ether ketone (PEEK, obtained from Invibio, Inc. was dissolvedin dichloroethane at a concentration of 1 μg/mL and then the solvent wasevaporated at 1 mbar pressure and 26° C. for 24 hours. FIG. 3A shows anSEM image of the surface structure after this process. The averagesurface feature size was 45 nm in diameter and 10 nm height. FIG. 3Bshows a comparable image from a control PEEK surface produced byevaporation of the solvent at atmospheric pressure and 26° C. for 24hours.

Polyurethane (PU, obtained from Sigma-Aldrich) was dissolved inchloroform at a concentration of 10 μg/mL and then the solvent wasevaporated at 0.5 mbar pressure and 26° C. for 48 hours. FIG. 4A showsan SEM image of the surface structure after this process. The averagesurface feature size was 10 nm in diameter and 5 nm height. FIG. 4Bshows a comparable image from a control PU surface produced byevaporation of the solvent at atmospheric pressure and 26° C. for 48hours.

Polyethylene (PE, Sigma-Aldrich) was dissolved in chloroform at aconcentration of 5 μg/mL and then the solvent was evaporated at 1.5 mbarpressure and 26° C. for 12 hours. FIG. 5A shows an SEM image of thesurface structure after this process. The average surface feature sizewas 10 nm in diameter and 5 nm height. FIG. 5B shows a comparable imagefrom a control PE surface produced by evaporation of the solvent atatmospheric pressure and 26° C. for 12 hours.

Example 4: Inhibition of Bacterial Growth on a Nanostructured PolymerSurfaces

The nanostructured and unstructured polymer materials described inExample 3 were tested for the growth on their surfaces of four differentbacterial species: Staphylococcus aureus, Staphylococcus epidermidis,Pseudomonas aeroginosa, and Escherichia coli. The procedure was asdescribed in FIG. 2, and the results are shown in FIG. 6.

For each of the materials (PEEK, PU, and PE) the nanostructuring process(“nanomodified” in FIG. 6) significantly (p<0.01) reduced the growth ofall tested bacterial species compared to the non-structured controls,

As used herein, “consisting essentially of” does not exclude materialsor steps that do not materially affect the basic and novelcharacteristics of the claim. Any recitation herein of the term“comprising”, particularly in a description of components of acomposition or in a description of elements of a device, can beexchanged with “consisting essentially of” or “consisting of”.

While the present invention has been described in conjunction withcertain preferred embodiments, one of ordinary skill, after reading theforegoing specification, will be able to effect various changes,substitutions of equivalents, and other alterations to the compositionsand methods set forth herein.

The invention claimed is:
 1. A method of nanostructuring a surface of apolymer material, the method comprising the steps of: (a) providing aliquid solution comprising a polymer material dissolved in an organicsolvent; and (b) evaporating the solvent from the liquid organic solventsolution low pressure sufficient for nanostructuring a surface of thepolymer material, whereby the polymer material solidifies into a solidform comprising the polymer material, wherein the surface of the polymermaterial is nano structured.
 2. The method of claim 1, wherein thepolymer material comprises a polymer selected from the group consistingof poly-lactic co-glycolic acid (PLGA), polyurethane, polycaprolactone,poly-ether-ether-ketone (PEEK), polyethylene, polypropylene,polypyrrole, and polystyrene.
 3. The method of claim 1, wherein thepolymer material comprises two or more different polymers.
 4. The methodof claim 1, wherein the solid form is a composite material comprisingsaid polymer material.
 5. The method of claim 1, wherein the organicsolvent is selected from the group consisting of chloroform, dimethylformamide (DMF), acetone, chlorobenzene, cyclohexanone, cyclopentanone,dimethyl sulfoxide (DMSO), nitrobenzene, 1, 2-dichlorobenzene,diisopropyl ketone, dioxane, ethylene chloride, isophorone, toluene, andmesityl oxide.
 6. The method of claim 1, wherein the low pressure is inthe range from about 0 to about 5 millibars.
 7. The method of claim 6,wherein the low pressure is about 2 millibars.
 8. The method of claim 1,wherein the nanostructuring inhibits adhesion of bacteria to thesurface.
 9. The method of claim 1, wherein the nanostructuring inhibitscolonization of the surface by bacteria selected from the groupconsisting of Pseudomonas aeruginosa, Escherichia coli, Staphylococcusaureus, and Staphylococcus epidermidis.
 10. The method of claim 1,wherein the nanostructured surface has a root mean square surface (RMS)roughness of about 100 nm or less.
 11. The method of claim 1, furthercomprising the step of: (a1) placing said organic solvent solutioncomprising said polymer material into a mold; wherein in step (b) thepolymer material solidifies into said solid form in the mold.
 12. Themethod of claim 1, further comprising the step of: (a1) applying saidorganic solvent solution comprising said polymer material to the surfaceof an article or a device.
 13. A method of nanostructuring a surface ofa polymer material, the method comprising the steps of: (a) providing anarticle or device comprising a polymer material surface and an organicsolvent capable of dissolving the polymer material; (b) applying thesolvent to the polymer material surface, whereby a portion of thepolymer material surface dissolves in the solvent thereby forming aliquid organic solvent solution comprising the polymer; and (c)evaporating the solvent from the liquid organic solvent solution at lowpressure sufficient for nanostructuring the portion of the polymermaterial surface, whereby the dissolved polymer material solidifies intoa solid form comprising the polymer material, wherein a surface of thepolymer material is nanostructured.
 14. The method of claim 13, whereinthe article or device has a shape that is retained during steps (b) and(c).
 15. A method of nanostructuring a surface of a polymer material,the method comprising the steps of: (a) providing a liquid organicsolvent melt comprising a liquid solution of a polymer materialdissolved in an organic solvent; (b) extruding the liquid organicsolvent melt from an extrusion device to form an extruded article; and(c) evaporating the solvent from the liquid organic solvent melt of theextruded article at low pressure sufficient for nanostructuring polymermaterial at a surface of the extruded article formed of the polymermaterial, whereby the extruded article solidifies into a solid formcomprising the polymer material, wherein the surface of the extrudedarticle formed of the polymer material becomes nanostructured.